Abstract : Synthetic-perturbation tuning--SPT--is a novel technique for assaying and improving the performance of programs on MIMD systems. Conceptually, SPT brings the powerful, mathematical perspective of statistically designed experiments (DEX) to the interdependent, sometimes refractory aspects of MIMD program tuning. Practically, SPT provides a needed reconfiguration mechanism via synthetic delays for what otherwise would be ad hoc, hand-tailored program setups for DEX. Overall, the technique identifies bottlenecks in programs directly as quantitative effects upon response time. SPT works on programs for both shared and distributed memory and it scales well with increasing system size.